And someone else traced it back to the Prophet, who said, "I must not find any of you reclining on his couch, when a matter I have commanded or forbidden reaches him, saying, 'I do not know. Whatever we find in the Book of Allah, we follow it.'"
حدثنا قتيبة حدثنا سفيان بن عيينة عن محمد بن المنكدر وسالم أبي النضر عن عبيد الله بن أبي رافع
وغيره رفعه قال لا ألفين أحدكم متكئا على أريكته يأتيه أمر مما أمرت به أو نهيت عنه فيقول لا أدري ما وجدنا في كتاب الله اتبعناه
Its isnad is sahih. Abu Dawud narrated it in the Sunnah, chapter "On Adhering to the Sunnah," no. 4605, and Ibn Majah, no. 13, from the hadith of Sufyan bin Uyayna, through the same route, and he explicitly stated that he heard it directly. Ibn Hibban graded it sahih, no. 98, and Hakim, 1/108, 109, said it meets the conditions of the two Shaykhs, and Dhahabi agreed with him.